Ignition high voltage switches (distributors) for internal combustion engines comprise a housing having a central bore which receives a shaft journaled for rotation in bushings axially aligned and supported within the bore. The housing has a stem portion for insertion into the engine and a base portion for supporting a cap. Installation of a completely assembled high voltage switch in the engine requires positioning of a gasket over the stem prior to insertion. The gasket provides a seal between the engine and base.
Prior art housings are composed of metallic material, most often aluminum. The benefits of using a plastic rather than a metal are threefold. First, no machining is required after the initial molding of a plastic part. Plastics can be injection molded into a final form. Metal castings often require subsequent machining, increasing the labor and tooling costs of producing the part. A metal ignition high voltage switch housing requires several additional manufacturing operations; in particular, machining the central bore for receiving the shaft, machining the outside diameter of the shaft for insertion into an engine bore and press-fitting bushings into the extreme ends of the central bore.
Second, plastics are much lighter than metals, reducing the total vehicle weight and improving fuel economy. Third, plastics are much less expensive than their metallic counterparts, reducing the material costs of producing the engine.
Ignition high voltage switch housings have continued to be manufactured from metals due to the high thermal temperatures and mechanical stresses to which they are subjected within the engine. High temperature, high strength insulating thermoplastics capable of withstanding these temperatures and stresses are available. Unfortunately, existing plastic injection molding technology has not been able to form a suitable housing. Several technological problems continue to exist. Particularly, molding of the long housing results in unacceptable warpage, making alignment of the two bushings located at extreme ends of the housing extremely difficult, as well as, control over the external dimensions of the stem necessary to fit properly within the engine bore.